PhD studentship available!

A funded PhD studentship is now available, starting October 2018, to work with my group in Durham. The project involves the investigation of skyrmions and is linked to The Skyrmion Project. Please contact me directly for more information.

Quantum field theory for the gifted amateur

A book by Stephen Blundell and myself, introducing the quantum theory of fields, is out now! Please see the book's websitefor further information.

Research Interests

News: A news item on the muon group's latest paper on the control of dimensionality in molecular magnets can be found here.

When atoms form a solid and electrons interact collective phenomena emerge. These phenomena include the phases of magnetic order, superfluidity and superconductivity, the emergence of new particles such as the magnon or the phonon and the occurence of topological objects such as kinks and vortices. Condensed matter physics is the investigation of this exotic world and provides the same fundamental insight into the Universe as the study of elementary particles or black holes.

I use muons to investigate condensed matter physics. Muons are subatomic particles that act as microscopic probes of magnetism. Subjects in which I'm interested include collective states of matter such as magnets, superconductors and glasses along with their excitations such as spin waves, vortices and diffusion. My work covers a wide range of scales from the quantum mechanical interaction of nuclei to the large scale dynamics of polymer chains.

Muons

Muons have a spin 1/2, which will Larmour precess in a magnetic field. They are also unstable and live for only 2.2 us (on average!). We detect their decay products and these tell us essentially which way each muon-spin was pointing at the moment of death. The technique we employ is known as muon-spin relaxation and involves stopping muons in materials where they precess until they decay. This tells us about the local magnetic fields in a material, making it useful for investigating magnets and the vortex phase in superconductors. Muons are produced using particle accelerators based at large facilities. I use the ISIS facility (http://www.isis.stfc.ac.uk/) in the UK, which is the world's most intense source of pulsed muons and neutrons and the Swiss Muon Source based at the Paul Scherrer Institut (http://www.psi.ch/).

Magnetism in reduced dimensions

Despite being one of the oldest discoveries in Physics, magnetism is relatively poorly understood. In some very beautiful systems, magnetic interactions may be constrained to act along a line of atoms (one-dimension) or in a plane of atoms (two-dimensions). These different dimensionalities are of fundamental importance and lead to exotic physical properties. An effective route to investigating these low-dimensional (i.e. 2D and 1D) phenomena is through the study of molecular magnets, which are self-assembled polymers formed through bridging paramagnetic cations (such as Cu2+) with organic molecular building blocks. The richness of carbon chemistry means that, in principle, molecular magnets can be nano-engineered to exhibit low dimensional properties.

Frustrated magnets

In some magnets interactions tend to oppose each other's influence and are therefore in competition. Such systems are said to be frustrated; it is not possible to satisfy all of the magnetic interactions to find the material's ground state. Materials showing these effects offer an insight into the factors that cause systems to adopt a particular ground state (such as permanently magnetic or disordered). One particularly exciting possibility for a frustrated system is the formation of a spin liquid state. This is a long sought after phase of matter which shows no long range magnetic order down to zero temperature, but which is nonetheless stable. In fact, we believe that we recently may have found such a state in an organic magnet!

Unconventional superconductors

Understanding unconventional superconductors is perhaps the most urgent problem in condensed matter physics. Many of the clues suggest that the behaviour of these materials is caused by a subtle interplay of magnetism and superconductivity. Muons are ideal probes of such systems since not only are we able to probe magnetism, but we may also use muons to map the field distribution within the superconducting vortex state allowing an accurate determination of the superconducting penetration depth. My interests currently lie in the recently discovered iron arsenide superconductors, where magnetism, structural distortions and superconductivity coexist across a rich phase diagram.